This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-136436, filed May 7, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a power converter apparatus using a power device, and more particularly to a power converter apparatus used in an inverter system for compressors and fan motors of air conditioners, refrigerators, etc., motors of washers, etc., and hydraulic control motors.
2. Description of the Related Art
In a conventional inverter system, a control section (e.g. MCU) for PWM (pulse width modulation) control and a power device (e.g. an IGBT (insulated Gate Bipolar Transistor) and a gate drive IC) for supplying current to the winding of a motor are coupled by a photocoupler. Thus, even if the potential of the winding of the motor has accidentally dropped to a ground potential level and shorted, the abnormality of the high-side device can be told to the control section from the high-side driver IC via the photocoupler. Upon receiving the information on the abnormality, the control section stops the ON-instruction to the power device. Thereby, the power device can be protected.
FIG. 1 is a circuit diagram showing the structure of a conventional power converter apparatus using photocouplers.
As is shown in FIG. 1, the power converter apparatus comprises a control section (MCU) M101; photocouplers PC101 to PC106; voltage power supplies V101 to V106; clocked inverters CI101 to CI106; IGBTs (Insulated Gate Bipolar Transistor) 101 to 106; and resistors R101 to R112. A load motor L101 is connected to the IGBTs 101 to 103.
The system shown in FIG. 1 has the following problems.
Since the system requires six photocouplers, the manufacturing cost is high. Since there is a delay in transmission time of the photocouplers PC101 to PC106, an error is great between the time of an order from the control section M101 and the time of execution of the order. Four power supplies V102 to V105 are necessary as control power supplies. Thus, the power converter apparatus shown in FIG. 1 is not preferable in terms of both the cost and performance of the system.
Under the circumstances, most of modern power converter apparatuses have recently adopted a microcomputer direct driving system (photocoupler-less system) and a single power supply.
FIG. 2 is a circuit diagram showing the structure of a conventional bootstrap-type power converter using a microcomputer direct driving system and a single power supply.
As is shown in FIG. 2, the power converter apparatus comprises a control section (MCU) M101; clocked-inverters CI101 to C109; voltage power supplies V101 and V106; IGBTs 101 to 106; transistors TR101 to TR103; diodes D101 to D103; capacitors C101 to C103; and resistors R113 to R115.
In this power converter apparatus, the high-side driving IC of the power device (IGBT 101, 102, 103) has the withstand voltage, which the photocouplers have to bear in the prior art. However, because of the problem of tolerable power, a control instruction from the control section M101 is sent to the high-side block by means of an edge pulse, and the edge pulse, in turn, is converted to a normal pulse within the high-side block.
As has been mentioned above, the transmission of signals from the low-side block to the high-side block within the high-side IC is effected by level-shifting using the edge pulse. Specifically, this is effected by short-time conduction of the high-withstand-voltage n-channel MOSFETs (TR101 to TR103). If the level-shifting n-channel MOSFETs (TR101 to TR103) (source-grounded) are turned on, current flows in the resistors R113 to R115 connected to the drains of these n-channel MOSFETs. At this time, voltage variations occurring between both ends of the resistors R113 to R115 are detected by the high-side block, and thereby signals are transmitted.
In general, in an inverter system used in a 100V commercial line, a voltage-doubler rectifier DC line is converter-controlled. Thus, the power device is required to have a withstand voltage of 600V, including a surge voltage. The above-mentioned level-shifting MOSFETs (TR101 to TR103) are also required to have a withstand voltage of 600V.
A high-withstand-voltage p-channel MOSFET is necessary for signal transmission from the high-side block to the low-side block. It is not possible, however, to realize a 600V p-channel MOSFET at a feasible cost.
Thus, when the high-side output section has shorted at a ground potential level, a large current flows. Even if the flow of large current is detected and a gate voltage to the IGBT is cut off by self-excess-current protection, the abnormality cannot be told to the low-side block. Consequently, the control section M101 is unaware of the short-circuit state of the high-side block at a ground potential level, and continues to send a turn-on instruction. In that event, enormous energy is applied to the high-side IGBT, leading to immediate destruction.
In the conventional system, the low-side power device (IGBT) can be protected against short-circuit to the voltage power supply. However, the high-side power device cannot be protected against short-circuit to the ground potential. The reason why the low-side power device can be protected against short-circuit to the voltage power supply is as follows. The low-side reference potential is equal to the reference potential of the control section. Thus, in the event of abnormality, information on the abnormality can be told to the control section from the low-side driver IC, and a turn-off instruction can be issued to the low-side power device.
However, if power to the high side is stopped when a large current in the high-side power device has been detected, the inverter system will halt even in the case of instantaneous flow of large current and the system will not function. Thus, this method cannot be used.
According to an aspect of the present invention, there is provided a power converter apparatus comprising: a detection circuit which detects whether a power device is in a short-circuit state; a control circuit which sets, when the detection circuit has detected that the power device is in the short-circuit state, the power device in an inoperable state for a predetermined time period, and restores the power device to an operable state after the passing of the predetermined time period; and a time period generating circuit which defines the predetermined time period for setting the power device in the inoperable state, by measuring a time from the detection of the short-circuit state by the detection circuit.